The object of this research is to further understand the mechanisms that vertebrates have evolved to survive the thermal stresses of the environment. Specifically, we will attempt to elucidate the mechanism that leads to the selection of cooler water in fish that have been cold acclimated. Preliminary results indicate that increased blood levels of triiodothyronine (T3) may be an important part of this mechanism. We will evaluate and extend this hypothesis by: (1) Evaluating the sensitivity of temperature selection to injections of T3 and thyroxine (T4) to alter temperature selection. (3) Evaluating the effect on temperature selection of heightened thyroid activity such as occurs during growth and smolting. (4) Evaluating the time course, during cold acclimation, of changes in plasma T3 concentrations and changes in temperature selection. (5) Establishing the importance of the brain in mediating the responses obtained in (4). When (if) adequate results have been obtained on parts (1) through (5), an effort will be made to quantify the neuro-transmitters involved in normal temperature selection in fish, the relationship of these transmitters to the alteration of temperature selection that occurs during cold acclimation and the cardiovascular-respiratory effects of intrahypothalamic injection of these neurotransmitters. Answers provided by this research will clarify an important aspect of thyroid function in fish, provide a possible mechanism for altered thermoregulatory behavior following cold acclimation, and elucidate the neurochemical basis of temperature selection in fish. These answers will aid in understanding how specific influences (such as occur during fever or hot flashes) produce alterations in the normally stable regulated body temperature in vertebrates.